Whole Oats. Whole oats have a hard outer hull that must be removed before it’s ready for human consumption. Removing the hard outer hull is not easy, so if you want whole oats to eat, purchase them already hulled where they are known as groats.
Oat Groats. Are the whole oat grain, with only the hard unpalatable outer hull removed, but with the kernel’s outer bran layer left intact. They are long and thin with a smooth shiny surface and look like brown rice. They can be eaten at this stage, but are typically processed into one of the forms below.
Steel-cut oats or Irish oats. Are made by passing groats through steel cutters which chop each one into three or four pieces. Since they still contain the whole grain including the oat bran, steel cut oats are very nutritious – but they do take longer to cook (about 25 minutes).
Rolled Oats. Are made by steaming groats and flattening them with a roller. These are sometimes referred to as old fashioned or thick oats. These are made by first steaming the whole groat for a few minutes, partially cooking it, then passing it between rollers to flatten it out. This variety of oats generally takes about 5 minutes to cook
Instant or Quick Oats. Are made in a similar fashion to rolled quick-cooking oats, except they are steamed longer and rolled more thinly. Instant oatmeal is typically packaged in envelopes with sweeteners, flavorings and other additives. It takes almost no cook time, just the addition of hot water. This is the most processed oatmeal and tends to have more calories (due to added sugars) per serving than unprocessed oatmeal. It also tends to be lower in fiber and other nutrients.
Oat Flour. Oats can be ground in to flour which usually comes in three grades – coarse, medium and fine. Medium oatmeal can be used in cakes and crumble toppings to give a nutty flavor, or added to soups as a thickener. Fine oatmeal flour adds a depth of flavor to bread and improves its shelf life due to the natural preservatives found in oats.
Oats gain part of their distinctive flavor from the roasting process that they undergo after being harvested and cleaned. Although oats are then hulled, this process does not strip away their bran and germ allowing them to retain a concentrated source of their fiber and nutrients.
For nutrition’s sake, whenever possible, stick to the old-fashioned or steel-cut oats. Old-fashioned oats require about 3 more minutes cooking time than the quick variety — well worth it especially when you can create your own flavor combinations and control what goes into your bowl.